It is customary to have an alternate power source for an installation if a primary, commercial power source is lost or becomes unreliable. The alternate power source may take the form of, for example, a generator driven by an internal combustion engine. When a transformation from the commercial power source to the alternate source is required, a transfer switch is used to disconnect the commercial power source and thereafter to connect the alternate power source to the load. The transfer switch conventionally provides an open, or break-before-make, transition between the two power sources. More specifically, the transfer switch disconnects the initial commercial power source before connecting the alternate power source to the load. The open transition switching avoids any potential surges that may occur as a result of the two unsynchronized sources applying power to the load simultaneously.
Open transition transfer switches to accommodate the precise application as described above are well known in the art. For instance, transfer switches employing a pair of circuit breakers and a mechanism for operating the two circuit breakers in opposition are a product of former patents. For instance, U.S. Pat. No. 3,778,633 to DeVisser, et al., issued on Dec. 11, 1973, entitled "Automatic Electric Power Source Transfer Apparatus," discloses a device that includes two circuit breakers mounted end-to-end and operated in opposition by a ganging member driven rectilinearly by a screw shaft engaging a traveling nut connected to the ganging member. Another transfer switch of this type uses two side-by-side circuit breakers with handles. The handles are engaged by clevises attached to a lever arm that is rotated about a pivot axis between the two circuit breakers. In still another transfer switch using two side-by-side circuit breakers, a plurality of slides engaging the circuit breaker handles are driven in opposition by gears having an eccentric pin to engage a camming surface on the slide.
The previously-described transfer switches provide open transition operation and thus momentarily interrupt power to the load. However, the open transition switch, where there is absolute and unabridged interruption of power to the load for a period of time, is unacceptable in certain situations. For example, a digital computer will lose the contents of its volatile memory if the power source is severed for even a very short interval. In yet another example, a central office with equipment serving a telephone network cannot be deprived of power for even an instant, or service is lost throughout the network.
To accommodate such critical applications, uninterruptible power supplies (UPSs) have been developed to provide continuous power to the computer or central office equipment should the commercial power source be lost. One embodiment of an uninterruptible power supply takes the form of a battery that is ordinarily charged by a charging device connected to the commercial power system. When commercial power source is lost, the battery provides direct current (DC) power to the load and an inverter coupled to and fed by the battery provides alternating current (AC) power to the load. Conventionally, a special static switch, located in proximity to the inverter, provides a rapid open transfer between the commercial power source and the inverter. The resulting interruption of power only lasts for a few milliseconds, thereby preserving the integrity of the power to the load and not adversely affecting the digital computer, central office equipment or other critical load.
Accordingly, what is needed in the art is an uninterruptible power supply that provides and maintains high quality output power even when transferring to a secondary input power source and employs essentially an automatic transfer switching method using a transfer switch without mechanical moving parts.